1. Technical Field of the Invention
The present invention relates generally to a vehicle power transmission device equipped with a plurality of power split rotors which work to split output power or torque among an electric rotating machine (e.g., a dynamo-electric machine), an internal combustion engine, and driven wheels of a vehicle and are designed to rotate in conjunction with each other and a power transmission control system for such a power transmission device.
2. Background Art
In recent years, in terms of reducing the amount of energy consumed by automotive vehicles, so-called hybrid vehicles have been put into practical use which are equipped with an electric rotating machine such as an in-vehicle power source functioning as both an electric motor and a generator in addition to an internal combustion engine. The hybrid vehicles are typically controlled to stop the internal combustion engine in a low speed running range in view of the fact that the internal combustion engine is usually inefficient in energy use at low speeds. However, the hybrid vehicles face difficulties in starting the internal combustion engine during running of the vehicles. For example, it is difficult to bring a rotor which is coupled with driven wheels of the vehicle and rotating at a relatively high speed into mechanical connection with the crankshaft of the internal combustion engine which is stopped.
In order to avoid the above problem, there have been in practical use hybrid vehicles equipped with an electric motor whose output shaft is connected directly to a crankshaft of the internal combustion engine to transmit the torque, as outputted from the electric motor, to the crankshaft to start the engine. After start-up of the engine, the torque, as produced by the engine, is transmitted to the driven wheels of the vehicle.
Additionally, there have been in practical use hybrid vehicles equipped with a typical planetary gear speed reducer (also called an epicycle reduction gear train) made up of three power split rotors: a sun gear, a carrier (also called a planetary carrier), and a ring gear which work to split power or torque among the electric rotating machine, the internal combustion engine, and the driven wheels of the vehicle. The driven wheels and the electric rotating machine are coupled mechanically to the ring gear. The generator is coupled mechanically to the sun gear. The internal combustion engine is coupled mechanically to the carrier. In operation, when torque is applied to the sun gear or the ring gear, the carrier is rotated, thereby rotating the rotating shaft (i.e., the crankshaft) of the internal combustion engine. The internal combustion engine is started by the output torque of the carrier. After the start-up of the internal combustion engine, the engine torque is transmitted to the driven wheels of the vehicle through the carrier.
For example, Published Japanese translation of International Patent Application No. 2004-514103 teaches the above type of power split rotors which split power between a main engine installed in the vehicle and driven wheels of the vehicle.
The direct coupling of the rotating shaft of the electric motor to that of the internal combustion engine, as described above, will cause the torque load to be exerted by the internal combustion engine on the electric motor when the internal combustion engine is not fired, but being free-wheeling or when the engine is being cranked by the electric motor, thus resulting in an increase in energy consumption in the vehicle. A problem is also encountered in that the pulsation of torque occurring at the rotating shaft of the internal combustion engine when started may result in a decrease in driveability of the vehicle.
Further, the use of the planetary gear speed reducer leads to the problem that starting of the internal combustion engine when the rotational speed of the carrier is low will cause the internal combustion engine to be kept run at a low speed for a while. This is against the intended purpose of the hybrid vehicles which is to run the internal combustion engine in a speed range in which the engine efficiency is high.